For the cutting of screw threads on the ends of pipes, a threading machine can be used which has a tool head into which the pipe can be inserted or which can be moved over the pipe end when the latter is in position. That tool head can carry a number of thread-cutting tools which can be shifted radially relative to the pipe on the tool holders forming radial slides. The tool holders and tools are generally uniformly spaced angularly about the axis of the tool heat and the pipe, i.e. the tool head and the pipe can be coaxial.
Appropriate actuating or displacement means can be provided on the tool head to shift the slides. Coaxial with the tool head or disposed along a common axis therewith can be a frame located on the side of the tool head turned toward the opposite end of the pipe from that being threaded and which can be coupled by the actuators on the tool head to the sliders or tool holders to insure a synchronous displacement of the sliders. The actuators can be, for example, rods or bars connected at one end to the frame and formed at their ends within the tool head with inclined teeth which actuate the rack and pinion systems which drive and couple the frame and rods to the sliders and hence the cutting tools.
An apparatus of this type has been described and illustrated in German patent document DE 44 38 818 A1.
The pipe thread is, as a rule, cut in a single pass of the tool head over the pipe end and the movement of the cutting tools and the head is generally controlled by a numerical control system. The machine is so configured that especially narrow or tight thread tolerances can be achieved. It is especially significant with such machines that the tool head itself is rotatable while the pipe is held stationary. In practice, as soon as the pipe is positioned at a particular location within the machine, the thread cutting advance of the tool head is effected.
The cutting tools, as has been noted, are uniformly angularly spaced on the cutting head and, while the number of cutting tools is optional, in the most common case six tools are provided, three of which are spaced angularly at 120° intervals while the other three, also spaced at 120° intervals are offset from the first, the two sets of tools being synchronized to move oppositely. This can be achieved by a central gear which meshes with the racks of the sliders formed by the tool holders.
At high rotary speeds of the tool head, which can reach 1000 rpm, the centrifugal force at the movable masses or components is very high and acts counter to the forces which must radially advance the cutting tools, thereby complicating the exact positioning of the tools. The gears which mesh with the racks of the sliders are configured as spur gears and serve in part to balance the centrifugal forces. A complete centrifugal force compensation, however, is achieved with this earlier system only when the sliders are in their central positions, i.e. in an intermediate position between extreme radial positions.
In order to provide a compensation for the centrifugal force when the sliders because of radial adjustment no longer are all at exactly the same radial spacing from the rotation axis and thus such that the centrifugal force does not act equally upon all of the sliders and cannot be balanced by identical positioning of the sliders, an additional centrifugal force balance system has been disclosed in the German Patent Document DE 101 33 856 A1. This compensating system is comprised of lobar balancing weights with toothed segments meshing with a gear that in turn meshes with the rack of a first slider. With the lobar balancing weight, however, the counter moment can be applied only through a relatively small swing. Larger differential centrifugal forces at the other sliders which do not have a balancing weight meshing with then through the intermediary of a respective gear can only be compensated through the counter moment at the first slider and at the central gear between the two sliders. In short, the centrifugal force compensation in this system has proved to be inadequate in spite of the additional provision of a balancing weight as described.